In an MR imaging apparatus, an examination object, usually a patient, is exposed within the examination space of the MR imaging apparatus to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei and the strength of the applied B0 field.
By transmitting an RF excitation field (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation field, the MR relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR relaxation signals which are emitted by the relaxation processes, are detected by means of an RF/MR receive antenna or coil. The received MR signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
Interventional and non-interventional instruments or medical devices are frequently used during the MR examination or treatment of an examination object and especially of a local zone or area thereof Such instruments or devices which are exemplarily mentioned above, are used for example for MR guided biopsies, thermal ablations, brachytherapy, vascular interventions, electrophysiology and other invasive or non-invasive applications.
A general problem with such instruments is that they usually comprise or are made of one or more elongate or longitudinal electrically conductive elements as explained above. Due to the fact that the interventional or non-interventional instruments are usually at least partly exposed to an RF/MR excitation field of an MR imaging apparatus when used during MR image generation, RF common mode currents can be induced in these elements by the RF/MR excitation field, so that the related adjacent or surrounding examination object is subject to a potential and undesired RF heating by these currents.
In other words, an elongate or longitudinal electrically conductive element in the sense above shall be any element in which RF common mode currents can be induced by the RF/MR excitation field, wherein these currents result in a potential and undesired RF heating of an adjacent or surrounding tissue of an examination object. In the following, all these elements shall be commonly denoted by the term “conductor”.
Generally, the above instruments have to be made RF-safe in the sense that any adjacent or surrounding tissue of an examination object is not RF heated to an undesired extent. The same applies for any RF transmission lines or cable the instruments comprise or by means of which the above instruments are connected with related RF transmitter units, MR receiver units, power supply units or other signal processing or control units.
In order to avoid or minimize such RF heating and to obtain RF-safety, various concepts have been proposed according to which conductors or RF transmission lines are provided for example with RF trap circuits, RF chokes, high impedance segments and/or other electrical circuits which are distributed along the length of the related conductor or RF transmission line in order to segment the same and by this to prevent the generation of RF common mode currents on the conductor by the RF/MR excitation field and to prevent the corresponding RF heating.
US 2009/0171421 discloses such an MRI/RF compatible medical interventional device which comprises a lead system with a first and a second conductor, wherein MR safety shall be obtained by providing the lead system along its length with a plurality of spaced apart circuit segments which have a high impedance at a high range of frequencies and a low impedance at a low range of frequencies.